The current LSI design rule reaches a finer size of the order of sub-quarter micron. Accordingly the light source for exposure becomes of a shorter wavelength. More particularly, the light source experiences a transition from g-line (436 nm) and i-line (365 nm) of a mercury lamp, which was the mainstream in the art, to KrF excimer laser (248 nm), ArF excimer laser (193 nm) or any shorter one.
In the manufacture of semiconductor devices (e.g., LSIs and VLSIs) and LC display panels, patterns are formed by exposing semiconductor wafers or LC glass substrates to light. The exposure step typically uses lithographic masks and reticles, which are collectively referred to as “exposure original plates”, hereinafter. Problems arise if debris deposits on the exposure original plate. Since the debris absorbs or deflects light, the transferred pattern may be deformed or provided with rough edges, and the underlying layer may be contaminated black. These adversely impact the size, quality, and appearance of the pattern.
Although lithographic operations are typically performed in a clean room, it is still difficult to always maintain the exposure original plate clean. It is then a common practice to mount a pellicle on the surface of the exposure original plate for dust-proof protection before exposure is carried out. Then foreign particles do not deposit directly on the surface of the exposure original plate, but on the pellicle. The foreign particles on the pellicle do not contribute to the image transfer as long as the focus of lithographic projection is at the pattern on the exposure original plate.
The pellicle is generally constructed of a pellicle frame and a transparent pellicle film. The pellicle film is stretched across and mounted to the frame at its top end, and the frame at its bottom end is attached to an exposure original plate via a pressure-sensitive adhesive (PSA) layer. The pellicle film is formed of nitrocellulose, cellulose acetate, fluoropolymers and similar materials which are fully transmissive to exposure light, for example, g-line (436 nm) and i-line (365 nm) of a mercury lamp, KrF excimer laser (248 nm), and ArF excimer laser (193 nm). The pellicle film is mounted to the frame by applying a good solvent for the pellicle film to the top end of the pellicle frame, placing the pellicle film in contact with the top end, and air drying until a bond is formed, or by adhesively bonding the pellicle film to the top end of the frame using acrylic resin, epoxy resin or fluoro-resin base adhesives. Disposed on the bottom end of the pellicle frame is a PSA layer of polybutene resin, polyvinyl acetate resin, acrylic resin or silicone resin which is capable of attaching the frame to the exposure original plate. If necessary, a liner is attached to the PSA layer for the purpose of protecting the PSA layer.
On use of the pellicle, it is attached to the exposure original plate by forcing them under pressure with the PSA layer sandwiched therebetween, so that the pellicle may enclose the pattern region on the surface of the exposure original plate. The pellicle film, the pellicle frame and the exposure original plate define a closed pellicle space. Since the pellicle is provided for the purpose of preventing debris from depositing on the exposure original plate, the pattern region of the exposure original plate is isolated from the ambient atmosphere outside the pellicle so that dust in the ambient atmosphere may not deposit on the pattern surface. Since the pellicle is provided for the purpose of preventing foreign particles from depositing on the pattern of the exposure original plate, the surfaces of the pellicle defining the closed pellicle space, that is, the lower surface of the pellicle film and the inner wall surface of the pellicle frame must be kept free of foreign particles.
Typically foreign particles on the pellicle film can be detected having a size of down to about 0.3 μm. By contrast, on the inner wall surface of the pellicle frame, only foreign particles with a size of about 10 μm or larger can be detected. This means that even when a pellicle inspection detects no foreign particles on the frame inner wall, there can be present foreign particles with a size of less than 10 μm. Such foreign particles below the detectable level may fall on the exposure original plate during operation.
To avoid such spalling, the inner wall of the pellicle frame is provided with an inner PSA layer as disclosed in JP-A S61-241756. Even when foreign particles of undetectable size are present on the inner wall of the pellicle frame, the inner PSA layer on the inner wall of the pellicle frame holds the foreign particles adhered and prevents them from falling off the frame.